The present invention relates to a method and apparatus for examining tissue in order to differentiate the examined tissue from other tissue according to the dielectric properties of the examined tissue. The invention is particularly useful to differentiate cancerous tissue, particularly breast cancer tissue, from normal, healthy tissue.
Breast cancer is the second leading cause of cancer deaths in women today (after lung cancer) and is the second most common form of cancer among women (after skin cancer). According to the World Health Organization, more than 1.2 million people will be diagnosed with breast cancer this year worldwide. The American Cancer Society estimates that in 2001, approximately 192,200 new cases of invasive breast cancer (Stages I–IV) will be diagnosed among women in the United States; and another 46,400 women will be diagnosed with ductal carcinoma in situ (DCIS), a non-invasive breast cancer. Though much less common, breast cancer also occurs in men, it being estimated that 1,500 cases will be diagnosed in men in 2001. It is further estimated that 40,600 deaths will occur in 2001 from breast cancer (40,200 among women, 400 among men) in the United States. The incidence rate of breast cancer (number of new breast cancers per 100,000 women) increased by approximately 4% during the 1980s but leveled off, to 100.6 cases per 100,000 women, in the 1990s. The death rates from breast cancer also declined significantly between 1992 and 1996, with the largest decreases being among younger women. Medical experts attribute the decline in breast cancer deaths to earlier detection and more effective treatments.
Mammography is currently the best available screening modality for early detection of breast cancer. If the mammography finds a subspecies legion, the individual is directed to undergo a biopsy or other advanced screening methods, like ultrasound or MRI CT etc. Only 20% of the women that undergo a biopsy proceed to a surgical treatment. The traditional method for histological confirmation involves open surgery biopsy. An alternative is image guided biopsy, which is less invasive and more costly. The total number of breast biopsies in the U.S. is about 1.2 M per year. The open biopsy itself is a surgical procedure in which the breast is open and the tumor or lump is taken out, preferably fully.
The traditional method of biopsy, however, is not always successful and fails to successfully remove the appropriate lesion in about 0.5–17% of the cases. Some of the reasons given for unsuccessful biopsies include: 1) poor radiological placement of the localization wire; 2) preoperative and intraoperative dislodgment of the wire; 3) surgical inaccuracy and inadequacy in excising the appropriate tissue; 4) failure to obtain a specimen radiograph; and 5) failure by the pathologist to locate the focus of the disease when searching through a larger tissue sample provided by the surgeon.
All of the above reasons stem from a fundamental problem that during the surgery, the surgeon does not have a real time indication or delineation of the tumor. Because of the difficulty in precisely delineating the cancerous tissue, the surgeon may cut out more than was really necessary to better assure that the entire tumor was removed.
Today, women with stage I and stage II breast cancer are candidates for treatment with modified radical mastectomy and with immediate reconstruction. Breast-conserving therapy (BCT) is also available. Breast conservation therapy consists of surgical removal of a breast nodule and of the auxiliary fat pad containing the auxiliary lymph nodes (about a quarter of the breast). This is followed by radiation therapy to the breast and auxiliary areas in some cases. In this type of operation, precise margin assessment or delineation of the cancerous tissue during the operation is crucial to the success of the procedure since the goal is to remove the tumor completely while minimizing damage to the breast.
This trade-off between complete removal of the tumor, and conservation of the breast, is usually difficult to optimize because the surgeon generally does not know the actual margins of the tumor. If the surgeon were able to clearly delineate the tumor margins during the operation by an on-line margin detector, this trade-off could be better optimized.
The ability of recognizing cancer cells, and especially breast cancer cells, using bioimpedance is well established in the biomedical literature5,6,7,8. The usual method for measuring bioimpedance is by introducing a sample into a special chamber and applying an AC current through it while recording the voltage across the sample at each frequency9,10. More modern methods rely on multiple electrode matrices which are connected with the human body and measure physiological and pathological changes. Some of the methods aim to localize tumor cells inside the human body and to form an image11,12. Although this method is approved by the FDA, it lacks the necessary accuracy for a screening device mainly because of the inherent limitations of long wavelengths and noise from the contact electrodes.
Another technique, based on magnetic13 bioimpedance, measures the bioimpedance by magnetic induction. This technique consists of a single coil acting as both an electromagnetic source and a receiver operating typically in the frequency range 1–10 MHz. When the coil is placed in a fixed geometric relationship to a conducting body, the alternating electric field in the coil generates electrical eddy current. A change in the bioimpedance induces changes in the eddy current, and as a result, a change in the magnetic field of those eddy currents. The coil acts as a receiver to detect such changes. Experiments with this technique achieved sensitivity of 95%, and specificity of 69%, distinguishing between 1% metastasis tumor and 20% metastasis tumor. Distinguishing between tumor and normal tissue is even better.
Although the exact mechanism responsible for tissue impedance at certain frequencies is not completely understood, the general mechanism14,15 is well explained by semi-empirical models that are supported by experiments16,17,18.
Variations in electrical impedance of the human tissue are described in the patent literature to provide indications of tumors, lesions and other abnormalities. For example, U.S. Pat. Nos. 4,291,708; 4,458,694; 4,537,203; 4,617,939 and 4,539,640 exemplify prior art systems for tissue characterization by using multi-element probes which are pressed against the skin of the patient and measure impedance of the tissue to generate a two-dimensional impedance map. Other prior techniques of this type are described in WO 01/43630; U.S. Pat. Nos. 4,291,708 and U.S. 5,143,079. However, the above devices use a set of electrodes that must be electrically contacted with the tissue or body, and therefore the contact is usually a source of noise and also limits maneuverability of the probe over the organ.
Other prior patents, for example U.S. Pat. Nos. 5,807,257; 5,704,355 and 6,061,589 use millimeter and microwave devices to measure bioimpedance and to detect abnormal tissue. These methods direct a free propagating radiation, or a guided radiation via waveguide, onto the organ. The radiation is focused on a relatively small volume inside the organ, and the reflected radiation is then measured. However, these methods lack accuracy and spatial resolution since they are limited by the diffraction limit.
Another prior art technique is based on measurement of the resonance frequency of a resonator as influenced by the tissue impedance. This technique also uses radiation from an antenna, usually a small dipole antenna attached to a coaxial line. Although non-contact, the device actually measures average values inside the organ, and its ability to detect small tumor is doubtful. Similar prior art is described in Xu, Y., et al. “Theoretical and Experimental Study of Measurement of Microwave Permitivity using Open Ended Elliptical Coaxial Probes”. IEEE Trans AP-40(1), January 1992, pp 143–150.3. U.S. Pat. No. 6,109,270 (2000 NASA) describes a measurement concept with a multi-modality instrument for tissue identification in real-time neuro-surgical applications.
Other known prior art includes an open-ended coaxial2,3,4 probe having a center conducting wire surrounding by an insulator and enclosed in an external shield. This type of tip generates both a near field evanescent wave and a far field propagating wave. The propagating wave is undesirable because it interferes with the near field evanescent wave. In order to minimizes the propagating wave, researchers attempted to use coaxial cable with smaller and smaller diameters. But eventually large energy losses and difficult construction limited this direction.
Other existing medical instruments provide general diagnoses for the detection of interfaces between different types of tissues, such as cancerous tissue and healthy tissue, etc. However, such detections have been limited clinically to pre-operative scans, or demand large scanning multi-million-dollar machines, like the MRI, CT, and Mammography. Furthermore, real-time attempts to use these detecting methods are very sensitive to movement of the body, and cannot really be used to position the cutting knife or the biopsy needle. Existing devices provide diagnostic data of limited use since the tissues sampled or removed, depends entirely upon the accuracy with which the localization provided by the preoperative CT, MRI, or US scan is translated to the intracranial biopsy site. Any movement of the organ or the localization device results in an error in biopsy localization. Also, no information about the tissue being cut by the needle or knife is provided.
Detecting breast cancer tissues by measuring bioimpedance is thus well established, and the ability of this technique for delineating cancerous cells inside the body has been proved. However, there is currently no reliable real-time bioimpedance measuring device of sufficiently high accuracy for local tissue characterization and of a spatial resolution comparable to that provided by mammography.